WASHINGTON (Reuters) - Regular exercise in adolescence and
young adulthood may help cut a woman's risk of developing
breast cancer before menopause, according to a U.S. study
published on Tuesday.

The women who were the most physically active were 23
percent less likely to develop premenopausal breast cancer than
the women who got the least exercise, the researchers wrote in
the Journal of the National Cancer Institute.

High levels of exercise from ages 12 through 22 contributed
the most to the protective effect, the researchers said.

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"The more activity, the greater the benefit," study leader
Dr. Graham Colditz of Washington University School of Medicine
and Barnes-Jewish Hospital in St. Louis said in a telephone
interview.

Previous studies showed that regular exercise in adulthood
leads to at least a 20 percent lower risk of breast cancer
after menopause, the researchers said. Research on exercise and
breast cancer risk before menopause had produced inconsistent
results, they said.

The researchers said the new study indicated women need
regular physical activity starting at a young age to comparably
lower their risk of breast cancer before menopause. They called
their study the largest and most detailed examination to date
of the impact of exercise on early breast cancer risk.

Colditz and colleagues studied 65,000 registered nurses
aged 33 to 51 who reported in 1997 how much leisure-time
physical activity they had done since the age of 12.

After six years of follow-up, 550 of the women were
diagnosed with breast cancer.

RUNNING OR WALKING

The women classified as most active did the equivalent of
running for 3.25 hours a week or walking for 13 hours a week.

"It's not marathon running. Any team sport will get you to
that level of activity -- and it doesn't even have to be a team
sport," Colditz said.

Women who exercised regularly -- but not as much as the
most-active group -- also had a reduced risk for breast cancer,
but not as much as the top group, Colditz said.

The benefit provided by exercise was not associated with a
particular sport or intensity, Colditz added.

About a quarter of all breast cancer cases are diagnosed in
women before menopause and these can be more aggressive and
harder to treat than breast cancer in older women.

Protection against breast cancer is just one of many
benefits of exercise, Colditz said. "It protects against
diabetes, heart disease, stroke. Clearly it's good for bones to
protect against osteoporosis and fractures. There is a clear
benefit across many of the chronic diseases."

He said there are several hormone-related hypotheses to
explain how exercise may cut breast cancer risk. A leading idea
is that physical activity can cut a woman's lifetime exposure
to estrogen, a hormone strongly implicated in breast cancer.

Breast cancer is the leading cause of cancer death among
women worldwide, killing an estimated 465,000 women annually,
according to the American Cancer Society. About 1.3 million
women are diagnosed annually worldwide.